For transmitting small amounts of data from a large number of nodes, e.g. sensor data such as heat, electricity or water meters, for example, to a base station, a radio transmission system may be employed. In this context, the base station receives (and possibly controls) a large number of nodes. Typically, more computing power and more complex hardware, i.e. generally also a receiver with increased performance, are available at the base station. Convenient quartzes, which in most cases have frequency offsets of 10 ppm or more, are used in the nodes.
There already exist various uni-directional and bi-directional systems for transmitting data between base stations and nodes. Known systems are, e.g., DECT (Digital Enhanced Cordless Telecommunications, an international standard for telecommunications by means of radio technology) and RFID (Radio Frequency IDentification). What is typical about these systems is that the base station specifies a reference frequency and a reference time to which the subscribers synchronize. In RFID systems, the RFID reader specifies a time window which directly follows its emission and within which the RFID transponders arbitrarily choose a point in time for their reply. The specified time interval is additionally split up into time slots of equal length. This is referred to as a time slot based (slotted) Aloha protocol. In DECT, in turn, times slots are provided within a fixedly specified grid. Here, the base station allocates an exact time slot to a subscriber which said subscriber is allowed to use for communication. Due to the inaccuracy caused by the quartz tolerance, a buffer time is provided between the time slots so that the telegrams do not superimpose on one another.
DE 10 2011 082 098 A1 describes a method of operating battery-powered data transmitters, in which method a sensor data packet (telegram) is split up into at least two data packets which are smaller than the actual information to be transmitted (telegram splitting). Telegrams are split up into several data packets (partial packets). Such a data packet (partial packet) is referred to as a hop. Within a hop, several information symbols are transmitted. The hops are emitted on one frequency or are spread across several frequencies (frequency hopping). In between the hops, there are breaks during which no transmission performed.
Data transmitters may not only be battery-powered, but they may draw their energy from the environment via an energy harvesting element such as via a solar cell, from a movement or from temperature differences, for example. For example, the EnOcean® system for wireless radio-controlled switches is known.
In data transmitters which draw their energy from the environment via an energy harvesting method, e.g. via light, temperature differences or vibrations, for example, a constant supply of energy may not be possible in some circumstances. The same may also apply to data transmitters which are battery-powered but wherein only a limited amount of energy may be withdrawn per time unit.